Adsorption of dimer molecules on triangular and honeycomb lattices

Grand canonical Monte Carlo simulations have been used to study the adsorpt ion of homonuclear dimers on triangular and honeycomb two-dimensional latti ces. Adsorption isotherms, adsorption energy, differential heat of adsorpti on, and configurational entropy were calculated for attractive and repulsiv e nearest-neighbor lateral interactions. In the second case (repulsive coup lings) and low temperatures, we found two novel ordered phases in the adsor bate: (1) low-coverage ordered phase (LCOP), with 2/5 (5/9) of the sites oc cupied for a triangular (honeycomb) lattice; (2) high-coverage ordered phas e (HCOP), with 2/3 of the sites filled for both lattices. The locations of the ordered phases are not symmetrical around theta = 0.5; this is a conseq uence of the nonequivalence between vacancies and particles in the case of adsorption with multisite occupancy. The influence of the ordered structure s on the thermodynamic quantities associated with the adsorbed monolayer ha s been analyzed and discussed in the context of the lattice-gas model.